Structure of a Palm Application, Part 3

Editors Note: In this third and final excerpt from "Palm OS Programming, 2nd Edition,"
Neil Rhodes and Julie McKeehan discuss how the operating system communicates with an
application when it's not running.

Other Times Your Application is Called

You know how an application works within the OS and now you know about the
organizational structure and content of the various source files used to create
an application. The last remaining piece of the puzzle is other times the OS
communicates with an application. Let's turn to the details of this.

The Palm OS makes a distinction between communicating with the active
application and communicating with a possibly nonactive application. In this
first case, the active application is busy executing an event loop, and can be
communicated with by posting events to the event queue. As shown earlier in our
example, this was how our application got closed; the appStopEvent
was posted to the event queue. When the
active application gets that event, it quits.

Because there are other times that your application gets called by the Palm
OS, there needs to be a way to communicate with it in those instances as well.
First, let's look at a partial list of the circumstances under which the system
might want to talk to your application:

When the user does a Find, the system must ask each installed application
to look for any records that match the Find request.

When beamed data is received, the system must ask the appropriate
application (the one that is registered to receive the data) to handle the
incoming item.

When a HotSync occurs, each application is notified after its data has been
synced.

After a reset, each application is notified that a reset has occurred.

If the system time or date changes, each application is notified.

If the country changes, each application is notified.

In all these cases, a communication must take place to a nonactive or closed
application. The question is: how does the system do this? The answer: its PilotMain routine is called with different launch codes.

Launch Codes

Within the Palm OS, it is the launch code that specifies to the application
which of the previously mentioned circumstances exists and what the application
needs to do. These codes arrive at the application's PilotMain
routine by way of its launchCode parameter. Here are some common launch
codes:

sysAppLaunchCmdFind

This code tells the application to look up a
particular text string and return information about any matching data. Called
by the system when the user does a Find.

sysAppLaunchCmdGoTo

This code tells the application to open if it isn't
already open and then to go to the specified piece of data. Called by the
system when the user taps on a found item.

sysAppLaunchCmdNormalLaunch

As we have already seen, this code opens the
application normally.

sysAppLaunchCmdSystemReset

Sent after a reset occurs.

TIP:
In the 4.0 OS and prior ones, some launch codes are sent to every installed
application (for example, after a sync, a reset, or a time change). When a user
has many applications installed, this broadcast can be slow. In the future,
they may only be sent to those applications that have registered for that
particular notification. This registration is available with a Palm OS 3.5 call
SysNotifyRegister.

Launch Flags

In This Series

Structure of a Palm Application, Part 2
In this second excerpt from "Palm OS Programming, 2nd Edition," Neil Rhodes and Julie McKeehan build on the terminology and conventions they introduced in the first excerpt and walk us through a sample application. This is the second in a series of three excerpts designed to familiarize developers with the structure of Palm apps.

The launch flags specify important information about how the application is
being executed. Here are some examples:

They may specify whether the application's global variables are available.
For performance reasons, globals are not available on many launch codes (this
can make the call to PilotMain very fast, as the
app's global variables don't need to be allocated or initialized). If globals
aren't available, it only makes sense that you can't read from or write to any
global.

They may specify whether the application is now the active application.

They may specify whether it had already been open as the active
application.

They may specify whether some other application is active.

Scenarios

To help clarify the relationship between the application and the times when
the system calls it, let's look at examples of when this happens and the flow
of code.

Find when another application is active

When the Memo Pad is the active application, and the user does a Find, the
Find Manager calls the PilotMain of every installed
application. When our application is called, the launch code is 1 (sysAppCmdFind), and the
launch flag is 0 (no globals, no UI).

Find when your application is active

Things happen differently when we do a Find with our application already
open. In this case, PilotMain is still called with
the same launch code: sysAppLaunchCmdFind, but now
the launch flag is different. It is 0x10 (sysAppLaunchFlagSubCall), specifying that the OReilly
Starter application is already open and running. This means that global
variables have been allocated and initialized, and we are running as an
indirect subroutine call from the application.

Figure 5-4 shows the stack trace
when the Find is done in this case. The stack trace shows that PilotMain is called (indirectly) from our AppEventLoop (which itself is called from our original
PilotMain).

Figure 5-4.The stack trace when doing a Find from within our own
application